Recent years have seen increasing demand for miniaturization and increased reliability of dielectric elements as electronic circuits reach higher densities, and miniaturization of electronic components such as laminated ceramic capacitors, together with increased capacity and higher reliability are rapidly progressing, while the applications thereof are also expanding. Various characteristics are required as this happens. Materials comprising barium titanate (BaTiO3) as the main component are often used conventionally as dielectric compositions for such applications.
For example, a smoothing capacitor or a snubber capacitor such as a motor vehicle DC-DC converter or AC-DC inverter is often used in a location in which a high DC bias of several hundred volts is applied. Dielectric elements used in these electronic components therefore must have a high dielectric constant when a high DC bias is applied. In addition, if the dielectric material has low mechanical strength, there is a risk of cracking or splintering etc. during production or during mounting on a substrate, and thus a risk of a product being non-conforming; high mechanical strength is also required at the same time.
There is therefore a problem with conventional electronic components having a dielectric layer comprising a dielectric composition which has BaTiO3 as the main component in that there is a reduction in dielectric constant when a high DC bias is applied. This problem is due to the fact that BaTiO3 is a ferroelectric material, which makes the problem difficult to avoid when BaTiO3 is used as the main component. When electronic components having a dielectric layer comprising a dielectric composition which has BaTiO3 as the main component are used for applications involving high DC bias application it is therefore necessary to devise a method for using such electronic components. According to one example of a known method, the amount of reduction in the dielectric constant is anticipated and a plurality of the electronic components is connected in parallel for use in order to maintain the required capacitance or dielectric constant.
Furthermore, when a conventional dielectric composition having BaTiO3 as the main component is used for applications under a low DC bias of several volts or less, the field intensity applied to the dielectric layers is small, so it is possible to adopt a design in which the dielectric layers are sufficiently thin and the electrode surface area is sufficiently small that breakdown does not occur. That is to say, the dielectric material can be made more compact and more lightweight. The required mechanical strength is also reduced if the dielectric material is smaller and more lightweight. For example, there is essentially no cracking or splintering even if the dielectric material is dropped during production because it is possible to maintain adequate mechanical strength which is commensurate with the size and weight of the dielectric material. However, when a conventional dielectric composition having BaTiO3 as the main component is used under a high DC bias of several hundred volts or greater, the dielectric layers must be sufficiently thick to ensure safety with respect to breakdown. As a result, it is necessary to increase the electrode surface area in order to maintain the required capacitance. That is to say, the dielectric material tends to become larger and heavier. As a result, the required mechanical strength also increases. The dielectric material may crack or splinter if it is dropped during production because it is not possible to ensure adequate mechanical strength which is commensurate with the size and weight of the dielectric material.
In order to solve these problems, Japanese Patent Application JP 2006-206362 A mentioned below describes a dielectric porcelain having barium titanate as the main component and containing Ca, Sr, Mg, Mn and rare earth elements, and characterized by a core-shell structure in which the Ca concentration is greater at the particle surface than at the centre of the particle, and the Sr, Mg, Mn and rare earth elements are unevenly distributed at the particle surface.
Furthermore, Japanese Patent Application JP 2005-22891 A mentioned below describes a dielectric porcelain characterized in that it comprises both perovskite barium titanate crystal grains in which part of the B site is substituted with Zr (BTZ-type crystal grains), and perovskite bismuth sodium titanate crystal grains in which part of the A-site is substituted with Sr (BNST-type crystal grains), and also characterized by a core-shell structure in which Mg, Mn and at least one rare earth element are present in the grain boundary phase between the BTZ-type crystal grains and the BNST-type crystal grains, and the mean particle size of both the BTZ-type crystal grains and the BNST-type crystal grains is 0.3-1.0 μm.
However, although a dielectric porcelain comprising BaTiO3 as the main component and having a core-shell structure such as that described in Japanese Patent Application JP 2006-206362 A has a relatively high dielectric constant value of 2500 or greater at 20° C. when a DC bias is not applied, the rate of change in the dielectric constant or the rate of change in capacitance (DC bias characteristics) when a DC bias of 5 V/μm is applied is a value less than −70%, so the rate of change is large and the value cannot be considered adequate for a capacitor which is used under a high voltage. Furthermore, there is no mention of the mechanical strength.
Meanwhile, a major feature of a ceramic composition such as that described in Japanese Patent Application JP 2005-22891 A lies in the fact that both BTZ-type crystal grains and BNST-type crystal grains having a core-shell structure are present. Furthermore, the BTZ-type crystal grains and the BNST-type crystal grains are both formed with a core-shell structure in which at least one of Mg, Mn and a rare earth metal is more unevenly distributed at the particle surface than at the centre of the particle, and as a result the dielectric constant at 20° C. when a DC bias is not applied achieves a relatively high value of 2750 or greater, and the DC bias characteristics when a DC bias of 3 V/μm is applied achieve a value of less than −20%, but this value cannot be considered sufficient for use under a high voltage, such as in a smoothing capacitor or a snubber capacitor such as a motor vehicle DC-DC converter or AC-DC inverter. Furthermore, there is no mention of the mechanical strength.